Springer Online Journal Archives 1860-2000
Abstract The charged hyperon beam at the CERN Super Proton Synchrotron (SPS) has been used to collect data on semileptonic decays of Σ−, Ξ−, and Λ. In this article we report results on the $$\sum \to ne\bar v$$ decay mode. A magnetic channel selected 100 GeV/c (130 GeV/c) negatively charged particles produced in the forward direction by interaction of the 210 GeV/c (240 GeV/c) SPS proton beam on a BeO target. The Σ− hyperons were identified in a DISC Čerenkov counter, and the negatively charged decay particle was analysed by a magnetic spectrometer. The neutron direction was obtained from the Σ− decay vertex and from the impact point measured in a neutron detector. Electron-hadron discrimination was achieved by the combined use of lead-glass and transition radiation detectors. The $$\sum \to ne\bar v$$ branching ratio measurement gave a value of (0.96±0.05) ×10−3. Measurements of the centre-of-mass distributions (baryon kinetic energy, electron-antineutrino correlation, and Dalitz plot distributions) yield the axial-vector to vector form factor ratio |g 1/f 1|=0.34 ±0.05. From the study of the electron momentum spectrum, the negative sign ofg 1/f 1, which is expected within the Cabibbo theory framework, is favoured by at least 2.6 standard deviations. The effects ofq 2-dependence off 1 andg 1 and of radiative corrections, the measurement of the weak magnetism form factorf 2, and the effect of possible second-class current terms are discussed. Results on the other Ξ−, Σ−, and Λ semileptonic decays are reported in separate articles.
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